The present invention relates to a substrate cleaning method and a cleaning apparatus for cleaning a substrate using an ultrasonically-agitated cleaning liquid being supplied to the substrate surface, without using a brush, or the like, contacted with the substrate surface.
For cleaning the surface of a substrate, there is a physical cleaning method in which a cleaning liquid such as pure water that is ultrasonically vibrated is supplied to the substrate surface. The cleaning method, which utilizes the cavitation phenomenon, has been used primarily in steps where a chemical cleaning cannot be used, e.g., in a cleaning step subsequent to the formation of metal wiring.
As electronic devices are miniaturized, there is a need to remove more minute contaminant particles and/or to perform the cleaning process without damaging the device. In order to meet the need, various apparatuses have been developed, including those that perform a cleaning process while an ultrasonic wave having a high frequency on the order of 1 MHz is applied to the cleaning liquid, and those with which the power of the ultrasonic wave to be applied to the cleaning liquid (hereinafter referred to as “ultrasonic output”) can be controlled within a low power range from some tens of watts to some hundreds of watts.
FIG. 14 is a schematic cross-sectional view illustrating a conventional ultrasonic cleaning apparatus (an apparatus that cleans the substrate surface by supplying an ultrasonically-vibrated cleaning liquid thereto).
As illustrated in FIG. 14, a substrate 101 is held by a vacuum chuck 102. The vacuum chuck 102 is spun by a motor 103. The substrate 101, the vacuum chuck 102 and the motor 103 are housed in an open-top cup 104. A nozzle arm 105 is provided outside the cup 104. The tip of the nozzle arm 105 is located above the substrate 101 within the cup 104. Moreover, an ultrasonic nozzle 106 for discharging a supplied cleaning liquid such as pure water toward the substrate 101 is provided at the tip of the nozzle arm 105. The ultrasonic nozzle 106 includes therein an RF vibrator 108 that is vibrated by an RF generator 107. Thus, the ultrasonic nozzle 106 can discharge the cleaning liquid such as pure water while ultrasonically vibrating the cleaning liquid.
FIG. 15 is a schematic plan view illustrating the scanning range of the ultrasonic nozzle 106 in the conventional ultrasonic cleaning apparatus illustrated in FIG. 14.
As illustrated in FIG. 15, a scanning range 109 of the ultrasonic nozzle 106 is set so as to extend in the radial direction between an edge position (a point along the periphery) 110 and a center 111 of the substrate 101 (being held by the vacuum chuck 102), which is a wafer. Thus, the ultrasonic nozzle 106 is reciprocated above the substrate 101 between the edge position 110 and the center 111.
FIG. 16 is a diagram illustrating a cleaning sequence using the conventional ultrasonic cleaning apparatus illustrated in FIG. 14.
The ultrasonic cleaning process is performed by supplying ultrasonically-vibrated pure water onto the surface of the substrate 101 from the ultrasonic nozzle 106 having a cleaning nozzle diameter of 4 mm while spinning the substrate 101 with the motor 103 at a rotational speed of 1000 rpm, as illustrated in FIG. 16. By using the nozzle arm 105, the ultrasonic nozzle 106 is reciprocated in the radial direction of the substrate 101 at a velocity of 40 mm/sec across the scanning range 109. In this way, the cleaning spiral pitch (the distance between the start and the end of a single turn in the spiral trace of the nozzle (the trace as viewed from the spinning substrate)) is sufficiently smaller than the cleaning nozzle diameter, thereby ensuring that the entire surface of the substrate 101 is cleaned with ultrasonically-vibrated pure water. Moreover, after the ultrasonic cleaning, the supply of pure water from the ultrasonic nozzle 106 is stopped, and the substrate 101 is spun by the motor 103 at a rotational speed of 4000 rpm to dry the substrate 101, as illustrated in FIG. 16. A cleaning method as described above is disclosed in Japanese Laid-Open Patent Publication No. 08-318235, for example.
With the conventional substrate cleaning method described above, the cleaning spiral pitch can be set to be smaller than the cleaning nozzle diameter, thereby suppressing the non-uniformity in the cleaning effect, by setting the substrate rotational speed to 1000 rpm, for example. However, the capacity for removing contaminant from the substrate is limited, and a sufficient contaminant removal rate cannot be achieved. In order to improve the contaminant removal rate, another method is proposed in the art, in which the number of times the substrate is scanned by the cleaning nozzle is increased. With this method, however, the cleaning time increases, thereby decreasing the device manufacturing capacity. In still another method proposed in the art, the contaminant removal rate is improved by increasing the ultrasonic output. With this method, however, the device may be damaged.